The most proven energy harvesting system utilizing natural energy in our surroundings is a system of obtaining electric energy from flow energy. The inventors of the present invention have ever proposed, as a power generation apparatus utilizing the system, a vibration power generation device that takes advantage of a longitudinal vortex excitation phenomenon (see, for example, Patent Document 1). This vibration power generation device has a first columnar body arranged such that a longer direction thereof intersects a flowing direction of a fluid and a second columnar body arranged such that a longer direction thereof intersects the first columnar body with a certain distance away from the same. The longitudinal vortex excitation is generated periodically from the vicinity of the intersection between the first columnar body and the second columnar body when the distance between the first columnar body and the second columnar body takes a predetermined value with respect to the diameter of the first columnar body.
On the other hand, a general method of obtaining electric energy from wind power or hydraulic power is turning a generator by rotating a wind turbine or a water wheel. Focusing on wind power generation in particular, propeller-type (horizontal axis type) is mainly used for large wind power generation apparatus. The principle of obtaining its rotational force is that when a propeller-type wind turbine is placed in the flow, an asymmetric flow field is formed around a blade, and thus a lift force is generated in the direction perpendicular to the flow, thus rotating the blade. In such an electric generator with large-scale wind turbine, there are provided reinforcing members inside blades with the increase in size of blades to achieve high output, so that the breakage or deflection of the blades caused by wind resistance and whirl is prevented (See, for example, Patent Document 2).
In addition, with a small scale wind turbine, as a result of having been inspired by wing veins of insects (e.g., dragonfly), veins of vegetation (for example, flying fruits of maple) and the like, there has been a proposal of providing thin blades with protrusions in the form of a leaf vein- or nervure-shaped pattern to increase the strength and performance of wings or blades (for example, see Patent Document 3).